Discovered accidentally by Rntgen in 1895. The primary mechanism for the production of x- rays is deceleration of a rapidly moving charge upon interaction with matter (bremsstrahlung). The x-ray spectrum from an x-ray tube consists of this continuous spectrum on which are superimposed narrow bands (characteristic radiation) that are a consequence of transitions between electronic energy levels of atoms. No sharp boundary exists between x- and ultraviolet radiation nor between x- and gamma radiation, although the latter term is usually restricted to radiation resulting from transitions between nuclear energy levels.

Boorse, H. A., and L. Motz, 1966: The World of the Atom, Vol. 1, 385401.

Reference:
American Meteorological Society Glossary of Meteorology Online